1. Technical Field
This disclosure relates to audio power amplifiers, systems, constant voltage generation circuits, and amplifier circuits for use in a speaker system, and in particular to such audio power amplifiers, systems, constant voltage generation circuits, and amplifier circuits, which employ a pop sound reduction circuit and reduce so called a pop sound likely generated when a power supply is turned on/off or a standby mode is terminated.
2. Discussion of the Background Art
It is well known that a speaker system sometimes generates a strange shock sound called a pop sound when a power supply is turned on/off or a standby mode is terminated and a voltage increases, i.e., in a transition period, at applicable section of the audio power amplifier circuit. Such a shock sound is not only considerably offensive to the ear and uncomfortable, but also sometimes damages the speaker.
In order to prevent such a pop sound, mute control is conventionally executed to forcibly stop signal transmission in the audio power amplifier during a time period starting from a start time when a power is supplied or a standby mode is terminated to a termination time when a power supply of the amplifier is sufficiently powered up and running. Another conventional approach is to slow down the change in voltage by employing a delay circuit formed from a capacitor and a resistor as shown in FIG. 10.
In other instances, a conventional audio power amplifier circuit does not take any countermeasures against the pop sound, and simply includes a power amplifier AMP1 connected to a differential amplifier circuit provided in an input stage. An audio signal IN is input to an inversion input terminal (i.e., a minus terminal) of the power amplifier AMP1 via a resistor R11. A feedback resistor R12 is connected between the output and the inversion input terminal of the power amplifier AMP1. A gain of the power amplifier AMP1 is determined by a ratio between the resistors R11 and R12. A reference voltage Vs is supplied to a non-inversion input terminal (i.e., a plus terminal) of the power amplifier AMP1 via a voltage follower AMP2. The output voltage SGout of the voltage follower AMP2 is utilized as a signal ground for an audio signal (hereinafter referred to as a signal GND or SG). A capacitor C12 is connected to stabilize the signal GND voltage between a signal GND (SG) terminal and ground.
A speaker is connected to the output terminal of the power amplifier AMP1 via a capacitor C11 that filters out a direct current component. A pop sound is likely generated when a power is supplied or a standby mode is terminated in the audio power amplifier circuit, because the signal GND voltage SGout quickly increases as shown by a dashed line in FIG. 12. A countermeasure can be taken to suppress the pop sound, as shown in FIG. 11. Specifically, the reference voltage Vs of FIG. 10 is replaced with a pair of resistors R13 and R14 and a capacitor C13. Thus, a voltage of a non-inversion input terminal of a voltage follower AMP2 and an output voltage SGout can be modified to slowly increase as shown by a dotted line in FIG. 12. As a result, the pop sound is reduced.
Further, Japanese Laid Open patent Application No. 10-261921 describes still another conventional technology directed to the same goal. Specifically, a power supply can indirectly be controlled to start and stop, by using a standby set signal to suppress a sharp transient change, so as to avoid uncomfortable pop sound while predictably avoiding breakdown such as caused by wiring short or the like.
However, the circuit of FIG. 11 requires a significantly large size capacitor C13 in order to suppress the pop sound to an insignificant level, and the capacitor C13 cannot be integrated on an IC, resulting in external attachment. Thus, when the capacitor C13 is employed in a small instrument such as a mobile phone, a headphone stereo, etc., largeness of the capacitor C13 impedes downsizing.
Further, in order to reduce the pop sound to such a level with the circuit of FIG. 11, it takes a long time (e.g. time elapsing T) from when a power is supplied or a standby mode is terminated to when an instrument becomes operable. On the other hand, if the capacitor C13 is downsized to reduce the time period before the operable condition, the pop sound is insufficiently suppressed, and is offensive to the ear.
Such a sound is now briefly discussed. In general, a sound comfortable to hear has a relatively low frequency, and can be heard if sound waves having different frequencies are superimposed in which one of them is a multiple or a simple integral ratio to the other. For example, a rale located at a center of a fingerboard of piano is generally tuned to approximately 442 Hz while a low rale lower by one octave is tuned to approximately 221 Hz. A high rale, on the other hand, one octave higher than the rale is tuned to approximately 884 Hz. When those rales are simultaneously hit, a considerably harmonious sound is generated because of superimposition of integral multiple sounds of the lowest rale as a reference.
In contrast, sound offensive to the ear, i.e., uncomfortable sound, can be heard when having a higher frequency. The same occurs when waves of different frequencies are intricately superimposed. For example, when a large number of fingerboards of piano are simultaneously hit at random, a ratio between respective frequencies is generally not a multiple or simple, thereby generating an uncomfortable sound. In this respect, if a sound quality of the pop sound is changed by either lowering a frequency of a sound wave or omitting sound waves having prescribed frequency components, uncomfortable pop sound can be suppressed.